1. Field of the Invention
The invention relates to a memory device, and particularly relates to a resistive random access memory (RRAM) device, a write verify method and a reverse write verify which are capable of saving power consumption, improving the performance of RRAM operations and improving the uniform performance for each RRAM cell.
2. Description of Related Art
RRAM devices are applied to a wide variety of electronic equipment, such as digital camera, smartphone, etc. To satisfy market's demands, the RRAM device is required to be small in volume and large in capacity, and is further required to have high speed, low power consumption and high reliability in storing information.
During a write or a reverse write operation on a RRAM cell, a verifying operation is performed to determine whether the resistance value of the RRAM cell reaches a target resistance value. When the resistance value of the RRAM cell reaches the target resistance value, the write or reverse write operation is successful. Otherwise, when the resistance value of the RRAM cell does not reach the target resistance value after a predetermined write or reverse write pulses (or a predetermined time period), the write or reverse write operation is failed.
However, in the write or reverse operation of the related arts, it is unable to determine a changing trend of the resistance value (e.g., the trend to reach the target resistance value). As such, even if the resistance value of the RRAM cell is changing in non-expected trend (e.g., increasing instead of decreasing as expected) after each write or reverse write pulse, the write or reverse write operation keeps applying the subsequent write or reverse write pulses. Accordingly, time and power consumption are wasted while the write or reverse operation is eventually failed, thereby reducing the reliability and the performance of the RRAM device.
Along with the popularity of RRAM devices, it is desirable to have a RRAM device, a write verify method and a reverse write verify method that are capable of determining the changing trend of the resistance value of each RRAM cell, thereby saving power consumption, improving the performance of RRAM operations and improving the uniform performance for each RRAM cell.